1. Field of the Invention
The present invention relates to a liquid crystal display device. More particularly, the present invention relates to a technique applicable to a drive circuit of a liquid crystal display device that is used as a display part of a portable device.
2. Description of the Related Art
Tin film transistor (TFT) liquid crystal display devices are widely used as a display device for personal computers, television sets, and the like. These liquid crystal display devices includes a liquid crystal display panel and a drive circuit for driving the liquid crystal display panel.
Among liquid crystal display devices of this type, small-sized ones are popular as a display device for portable devices such as cellular phones. A liquid crystal display device that is used as a display device of a portable device needs to be small in size and yet high in definition.
Generally speaking, enhancing the definition of a liquid crystal display device increases the wiring length, reduces the wiring width, and causes other phenomena that raise the wiring resistance value. As a result, at a location distant from the drive circuit, the electric potential falls due to a drop in voltage and the signal waveform is dulled.
JP 06-004046 A (hereinafter, referred to as Patent Document 1) describes a liquid crystal display device that varies the applied voltage depending on the position of a relevant scanning signal line. Patent Document 1, however, merely touches upon varying the applied voltage and does not mention a circuit that corrects the applied voltage depending on the position.
Liquid crystal display devices as a display device for portable devices are requested to be further enhanced in definition. Enhancing the definition of a high-definition liquid crystal display device means even longer wiring lines of the liquid crystal display panel, even narrower wiring width to keep the aperture ratio high, and an even larger wiring resistance value.
In conventional liquid crystal display devices, inverse display voltages are applied to adjacent pixel electrodes while the common voltage (the voltage of a counter electrode) is kept constant. For further lower voltage driving, conventional liquid crystal displays employ “common alternate driving” (polarity inversion) in which the common voltage is changed as well toward a polarity reverse to that of voltages applied to pixel electrodes.
Despite the fact that the common voltage in common alternate driving frequently switches between the positive polarity and the negative polarity, counter electrode signal lines which supply the common voltage are limited to a narrow wiring width, and the counter electrode signal lines having a narrow wiring width raise a problem in that the counter electrode voltage is not steady depending on the level of voltages written to pixel electrodes or the length of the signal lines.
Specifically, in common alternate driving, a single counter electrode signal line supplies a positive polarity or negative polarity common voltage to all pixels that constitute a row that is being scanned for the duration of the scanning of the row.
In this type of driving, when the number of pixels in the lateral direction is high, the amount of electric charges supplied by a single counter electrode signal line can exceed the supply capacity of the signal line. On the other hand, increasing the number of pixels in the longitudinal direction while keeping the frame frequency constant shortens the scanning time per row and there is not enough time to supply sufficient electric charges from a single common wiring line. This also increases the wiring resistance of counter electrode signal lines and accordingly worsens the problem in that a change in pixel electrode voltage changes the common voltage at a location distant from the drive circuit.
In addition, common alternate driving where the polarity of the common voltage is frequently inversed puts heavy load on the drive circuit. Another problem is that, in data signal lines, too, an increase in wiring resistance of data signal lines causes a drop in voltage at a location distant from the drive circuit.
Fundamentally, pushing a high resolution to a higher level requires supplying more current in a shorter period of time, and it is therefore necessary to widen the wiring width and thereby lower the wiring resistance in order to keep a change in common voltage small enough to cause no problems in a displayed image. On the other hand, the aperture ratio has to be satisfactorily high, and obtaining a high aperture ratio requires the contrary, i.e., narrowing the wiring width of counter electrode signal lines and other wiring lines.